Method for solid-phase microextraction and analysis, and a collector for this method

ABSTRACT

The invention relates to a method for solid-phase microextraction and analysis of substances in a carrier fluid, in which a collector is brought into contact with the stirred fluid containing the substances for a sufficient time and is then subjected to a solid-phase extraction directed at at least one substance adhering to the collector, and desorbed substances are transported for analysis by means of a carrier gas, in which the carrier fluid containing the substances is stirred in a receptacle of a magnetic stirrer by means of a coated magnetic stirring element as the collector, and/or the carrier fluid is made to move intimately relative to the collector by means of ultrasound, and then the stirring element is placed in a solid-phase extraction device.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of co-pending applicationSer. No. 09/524,682, filed Mar. 14, 2000, the contents of which areincorporated herein in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method for solid-phase microextraction andanalysis of substances originating for example from a carrier fluid andto a collector for use within this method. The invention also relates toa collector, which serves as a passive collector in a gaseous, forexample loaded environment containing substances to be analyzed and amethod for solid-phase microextraction and analysis of these substances.

2. Description of the Related Art

A method of this type is known from Boyd-Boland et al., Environ. Sci.Technol. Vol. 28, No. 13, 1994, 569A-574A and from EP 0,523,092 B1, inwhich a special syringe is used, which has a fiber which can betelescoped through the syringe needle. The fiber, which is expedientlycoated, is brought into contact with the carrier fluid which containsthe substances to be examined and at the same time is being stirred,after which the fiber is retracted and the syringe needle is introducedinto a feeding device of an analyzer, followed by desorption of adheringsubstances using a carrier gas. The fiber has only a very limitedabsorption capacity for substances which are to be examined and,moreover, is only dipped into the stirred carrier fluid, so thatconsequently the sensitivity of the analysis itself leaves something tobe desired if the coated fiber is vibrated. In addition, it is knownfrom DE 196 19 790 C2 to have the microfiber functioning as a collectorrotate about its own axis by means of an electric motor with arotational speed of one's choice.

SUMMARY OF THE INVENTION

An object of the invention is to provide a method for solid-phasemicroextraction and analysis of substances to be analyzed andoriginating for example from a carrier fluid, which provides asignificantly improved sensitivity.

Another object of the invention is to provide a collector forsolid-phase microextraction and analysis of substances to be analyzedand originating for example from a carrier fluid.

A further object of invention is to provide a collector which may beused within the method for solid-phase microextraction and analysis ofsubstances.

A still other object of the invention is to provide a collector as apassive collector for solid-phase microextraction and analysis ofsubstances for example pollutants to be analyzed and originating forexample from an environment.

A still other object of the invention is to provide a method forsolid-phase microextraction and analysis of substances to be analyzedand originate from an environment, for example pollutants.

A subject of the invention is a method for solid-phase microextractionand analysis of substances in a carrier liquid, in which a collector isbrought into contact with the stirred liquid containing the substancesfor a sufficient time and is then subjected to a solid-phase extractiondirected at at least one substance adhering to the collector, anddesorbed substances are transported for analysis by means of a carriergas, wherein the carrier liquid containing the substances is stirred ina receptacle of a magnetic stirrer by means of a coated magneticstirring element as collector, and/or the carrier liquid is made to moveintimately relative to the collector by means of ultrasound, andafterwards the stirring element is arranged in a solid-phase extractiondevice.

Another subject of the invention is a collector for the solid-phasemicroextraction and analysis of substances to be examined, in particularfor use in a thermal desorption apparatus of a gas chromatograph,comprising a carrier made from magnetic material, which is suitable as astirring element for a magnetic stirrer and is provided with a sorbentand/or adsorbent coating for the substances to be examined.

A still other subject of the invention is a coated magnetic collector,which serves as a passive collector in a gaseous, for example loaded,environment containing substances to be analyzed.

A still other subject of the invention is a method for the solid-phasemicroextraction and analysis of substances in an environment, in which acollector is exposed to the environment for a sufficient time as apassive collector and then the collector is positioned in a solid-phaseextraction device, desorbed substances being transported for analysis bymeans of a carrier gas.

The use of a stirring element which is actuated by a magnetic stirrerand/or an ultrasonic agitator increases the accuracy of analysis veryconsiderably, and moreover it is possible to use large-volumereceptacles, for example liter receptacles, for the fluid containing thesubstances to be examined.

Further objects, advantages and embodiments of the invention are evidentfrom the following description.

The invention is explained in more detail below with reference toexemplary embodiments of the invention, which are illustrated in theattached drawings, of a device for carrying out the method and ofpassive collectors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view of a device for carrying out the methodfor solid-phase microextraction and analysis of substances which are ina carrier fluid.

FIGS. 2 to 4 show various embodiments of passive collectors, in section.

FIG. 5 shows a comparison of two equilibrium curves relating to theprior art and the invention.

FIG. 6 is a diagrammatic view of a further device for carrying out themethod for solid-phase microextraction and analysis of substances whichare in a carrier fluid.

FIG. 7 is a diagrammatic view of a headspace receptacle.

FIG. 8 diagrammatically depicts a device for carrying out the method forsolid-phase microextraction and analysis of substances which are in acarrier liquid.

FIGS. 9 to 11 show various embodiments of collectors for solid-phasemicroextraction and analysis of substances which are in a carrierliquid, in section.

FIG. 12 shows a comparison of the yield with regard to a methodbelonging to the prior art (SPME method) and the present method(headspace sorptive extraction method: HSSE method).

FIG. 13 shows a GC spectrum of substances which are in a carrier liquidusing the present method.

FIG. 14 shows a gas chromatography spectrum (GC spectrum) for substanceswhich are in a carrier liquid using the SPME method.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

In accordance with FIG. 1, a magnetic stirrer 1 is provided, whichcomprises a receptacle. 3 which is positioned on a base 2 and is in theform of a beaker and may expediently be closed at its top side by aseptum 4. Before it is closed by the septum 4, the receptacle 3 receivesa carrier fluid containing substances which are to be analyzed. Thereceptacle 3 may have been filled and closed in advance at a samplingpoint. The carrier fluid may be water and/or an organic solvent or amixture thereof or liquefied gas.

The base 2 comprises an electric motor 5, the shaft of which bears amagnet 6 eccentrically. In the receptacle 3, there is a stirring ball 7made from ferromagnetic material, such as iron or steel, which isadvantageously glass- or plastic-sheathed and has a diameter in therange of a few millimeters. The plastic sheathing 7 a used may, forexample, be polytetrafluoroethylene or another fluorinated hydrocarbonpolymer. The stirring ball 7 is preferably covered with an active phase7 b for sorption/adsorption of substances contained in the carrierfluid. This may be a coating selected from the group comprisingpolyethylene glycol, silicone, polyimide, octadecyltrichlorosilane,polymethylvinylchlorosilane, liquid-crystal polyacrylates, graftedself-organized monomolecular layers and inorganic coating materials.

The stirring ball 7 can be stirred for a sufficient time during which itcomes into intensive contact with the carrier fluid and therefore withthe substances contained therein and sorbs and/or adsorbs the latter, sothat it serves as a collector. After the end of stirring, the stirringball 7 is picked up and introduced in a solid-phase extraction device,preferably a desorption device 8. The latter advantageously comprises adesorption tube 9 with a diameter section whose diameter is slightlygreater than that of the stirring ball 7, followed, via a frustoconicalsection, by a diameter section whose diameter is smaller than that ofthe stirring ball 7. The desorption device 8 is part of an analyzer 10,for example of a gas chromatograph, connected to a carrier gas port 11,so that carrier gas flows through the desorption tube 9 passing thestirring ball 7, desorbing substances adhering thereto, and can supplythese substances for analysis. The desorption device 8 preferablycomprises a heating device 12, in order to carry out a thermaldesorption.

The stirring ball 7 can be removed from the receptacle 3 automaticallyby means of a discharging device 13 which penetrates the septum 4 andmay be designed in the form of a gripping device, suction device or alsoas a magnet, and can be positioned in the desorption tube 9, which canthen be placed automatically in the desorption device 8, so that theentire solid-phase microextraction and analysis can be performedautomatically. For this purpose, an appropriate receptacle 3 for aplurality of samples can be arranged on a turntable which can rotate insteps and beneath which, in one position, the base 2 of the magneticstirrer 1 is arranged.

In order to achieve reproducible measurements, stirring times ofapproximately 45 to 60 minutes are generally required.

As an alternative to a stirring element in the form of a stirring ball,it is also possible to use an element in the form of a stirring rod 14.This may have a coated rod-like carrier 15 made from ferromagnetic orparamagnetic material; in the latter case, it should have a minimumlength of approximately 2 cm, while shorter lengths are also possible ifferromagnetic material is used. For example, it may be a rod-likecarrier 15 which is rounded at the ends and is entirely coated with theactive phase 15 b (FIG. 3) or also a carrier comprising a section ofwire which is surrounded by a cylindrical sheath 16 made from materialof the active phase which is in the form of a flexible tube (FIG. 4). Byway of example, the rod-like carrier 15 may have a diameter ofapproximately 3 to 6 mm.

It is also possible to achieve improved analysis if the carrier fluidwhich contains the substances and is situated in a receptacle containinga collector, which is preferably a stirring ball 7, is alternatively oradditionally set in motion by means of ultrasound. FIG. 6 shows oneembodiment of such an ultrasonic stirring device as a resonator unit 17in which one or more ultrasonic generators (vibrators) 19, which areshielded by means of insulating plate 18, arc incorporated at the bottomand/or on the sides of a housing 20. A metal wall 21 which is mounted infront of the ultrasonic generator 19 and has a wall thickness of d=n*λ/2(λ=acoustic wavelength) transmits the vibrations to a coupling liquid 22in the resonator unit 17, preferably water, which is set in motion. Thereceptacle containing the substances to be analyzed is introduced intothe resonator unit 17.

In this case, the receptacle 3 is also advantageously one of a magneticstirrer, in which case stirring is carried out by means of the coatedmagnetic stirring ball 7 as collector, so that the ultrasound is appliedin addition to the magnetic stirring.

In general, thermal, liquid or desorption by means of supercriticalgases is possible.

As an alternative to being introduced into a thermal desorption device8, the stirring element can be arranged in a headspace receptacle 23(FIG. 7), the diameter of which is only slightly greater than thediameter of the stirring ball 7, by means of the discharging device 13.The headspace receptacle 23 is then closed by means of a septum 24 and asealing ring 25, using a closure instrument, and is introduced into aheadspace head 26. In the latter, the headspace receptacle 23 ispreheated by means of a heating device 27 and there is a build-up ofpressure, during which an equilibrium for the volatile substances to beexamined with the gas phase 28 above the stirring ball 7 is established.These substances can be removed by means of a syringe which penetratesthe septum 24 and can be fed to the separation column of, for example, agas chromatograph.

As an alternative to being desorbed in a thermal desorption device 8,the stirring element may also be introduced into an extraction devicecontaining an organic liquid, the organic liquid used exhibiting a highlevel of interaction with the substances to be examined and absorbingthe latter—if necessary during a stirring movement of the stirringelement with respect to this liquid, after which the liquid which isenriched with the substances to be examined is taken up by means of asyringe and taken to a feeding device of, for example, a gaschromatograph, in order to be fed for analysis, for example using agas-chromatography separation column, by means of a carrier gas.

Due to the use of a stirring element in a magnetic stirrer or,alternatively or additionally, in an ultrasonic agitator and itsintensive contact with the carrier fluid containing the substances to beexamined, it is possible to achieve a sensitivity of analysis which isorders of magnitude, for example about 1000 times, butter than the useof the known fiber. FIG. 5 shows a diagram comparing the yield (plottedon the ordinate) of absorbed substances for a known fiber coated withactive phase (curve A) and a stirring rod which is sheathed by activephase and has been stirred by means of the magnetic stirrer according tothe invention (curve B) at mass equilibrium, the concentration quotient(K(o/w)) of a substance in octanol and water being plotted on theabscissa. This coefficient (for normal temperature) can be found in theliterature for a wide range of substances. For example, if thisconcentration quotient is 100, it can be seen from the diagram shown inFIG. 5 that in this case the coated fiber provides a yield ofapproximately 1% and the invention provides a yield of approximately50%. At a concentration quotient of below 100, the coated fiber cannotgenerally carry out any reliable measurement, while the sheathedstirring element generally still allows absolute reliable measurements.Generally, the covered stirring element improves the measurementaccuracy considerably, i.e. by powers of 10, and widens the measurementrange considerably, in that the sensitivity of analysis is improvedapproximately by a factor of 1000. With the sheathed stirring element,there is generally no need for the sensitivity of analysis to beimproved by heating the liquid containing the substances to be examined,as is necessary in many cases for a coated fiber and also causesmeasurement errors.

Moreover, a stirring element of this type may be arranged as a passivecollector in a gaseous, for example loaded, environment containingsubstances to be examined or may be carried by a person working in theenvironment, in which case the passive collector is exposed to theenvironment for a sufficient time and afterwards the substances which ithas sorbed and/or adsorbed are subjected to extraction, after whichdesorbed substances are transported for analysis by means of a carriergas via a feeding device, for example in-order to monitor personalexposure to pollutants.

In the heatable headspace extraction apparatus 100 illustrated in FIG.8, there is a headspace vessel 102, which is expediently closed off atthe top by a headspace vessel cap 103 and by means of a septum 104, theheadspace vessel 102, before it is closed off, receiving a carrierliquid 105 containing substances which are to be investigated and may bein a polar form, of medium polarity or, if appropriate, may even bepolar. The carrier liquid may be water and/or an inorganic solvent orsolvent mixture or liquefied gas.

The headspace extraction apparatus 100 includes a heating apparatus 106,which can be used to heat the contents of the headspace vessel 102. Anequilibrium of the volatile compounds is established between the carrierliquid 105 and a gas phase 107 above it. When heated, additionalpressure is built up inside the headspace vessel 102. In the gas phase107 above the carrier liquid 105 there is a collector 108 whichcomprises an inert support l 09, for example a rod-like glass tube(closed off from the outside at least during the headspace extraction),and a hose 10 which has been pushed on, at least partially surrounds theglass tube and comprises hydrophobic material as active phase. The hose10 is used for the sorption of substances which are situated in the gasphase 107, are derived from the carrier liquid 105 and are to beinvestigated, the analysis of which can then be carried outsubstantially without water. The material of the hose 110 comprises, asactive sorption phase, by way of example substances selected from thegroup consisting of polydimethylsiloxane, polyethylene glycol, silicone,polyimide, polymethylvinylchlorosilane, octadecyltrichlorosilane,liquid-crystal polyacrylates, grafted self-organized monomolecularlayers and inorganic coating materials, or consists of these materials.

The collector 108 is exposed to the gas phase 107, above the carrierliquid 105, for a sufficient time, and therefore exposed to thesubstances which are to be investigated, of which there is then anincreased level in the hose 110 of the collector 108, which has asorptive action on the substances. On account of the considerablethickness of the hose 110, the substances which are to be investigatedcan accumulate in the hose 110 until virtually complete saturation,since the equilibrium of the generally volatile substances which isestablished between the carrier liquid 105 and the gas phase 107 can beshifted towards the gas phase on account of permanent sorption of thesesubstances in the active phase of the collector 108.

After the sorption or a predetermined time has ended, the collector 108is picked up and is placed in a solid-phase microextraction apparatus,preferably a desorption apparatus 111. The latter expediently comprisesa small desorption tube 112 with a diameter section whose diameter isslightly greater than that of the collector 108 and which is adjoined,via a frustoconical section, by a diameter section whose diameter isless than that of the collector 108. The desorption apparatus 111 isconnected, as part of an analysis unit 113, for example a gaschromatograph, to a carrier gas connection 114, so that carrier gas canflow through the desorption tube 112 past the collector 108, so thatsubstances adhering thereto are desorbed and can pass these substancesfor analysis. The desorption apparatus 111 preferably comprises aheating device 115, in order to be able to carry out thermal desorption.

The entire method for carrying out the solid-phase microextraction andanalysis of substances in a carrier liquid can be fully automated, bythe collector 108 being automatically removed from the headspace vessel102 with the headspace vessel cap 103 and the septum 104 by means of atransfer member 116, which may be designed in the form of a gripper,sucker or as a magnet, and transferred into the desorption tube 112. Thedesorption tube 112 is then likewise automatically placed in thedesorption apparatus 111, so that the entire solid-phase microextractionand analysis can be performed automatically.

For this purpose, for a plurality of samples corresponding headspacevessels 102 may be arranged on a turntable which can be rotated in stepsand next to which, in one position, the headspace extraction apparatus100 and in a further position the transfer member 116 are arranged.

To enable reproducible measurements to be carried out, sorption times ofa few minutes up to 60 min are generally required, depending on thevolatility of the substances to be investigated and therefore anyheating operations which may additionally be required.

In the embodiment of the collector 108 illustrated in FIG. 9, the inertsupport 109 is in the form of a rod and is at least partially surroundedby the hose 110 which has a sorptive action.

However, the collector 108 may also be provided with a ferromagnetic orparamagnetic support 109 which is surrounded by an inert casing of, forexample, glass in which case the hose 110 at least partially surroundsthe inert casing 117, as illustrated in the embodiment shown in FIG. 10.The collector 108 may then be detachably secured to a separate magneticor magnetizable holder (not shown). However, any other type ofdetachable holder using a plug connection, a screwed connection, or anyother suitable connection for detachably securing it to the collector108 is possible, for example if a magnetic support 109 is not used.

The inert support 109 preferably has a minimum internal diameter of 0.8mm, in particular has an internal diameter of 1.0 to 1.3 mm, since thisminimum internal diameter is expedient in order to provide sufficienthose material for sorption.

In the embodiments of the collector 108 with a ferromagnetic orparamagnetic support 109, it is preferable to use a transfer member 116with a magnetic action.

In the further embodiment illustrated in FIG. 11, the collector 108 isformed integrally with the headspace vessel cap 103. This unit can befitted onto the desorption tube 112 so as to close off the latter in asealed manner, by means of the transfer member 116. The hose 110virtually completely surrounds the inert support 109.

Depending on the nature of the substances to be investigated, the hose110 should have a length of preferably 5 to 60 mm, in order to providesufficient sorption material.

EXAMPLE AND COMPARATIVE EXAMPLE

The following examples demonstrate the advantages of the present HSSEmethod over the known SPME method.

An analysis using the HSSE method was carried out under the followingconditions: 40 ppb of a solvent mixture (C6-C13) were added to 10 mL ofan aqueous sample and the mixture was introduced into a 250 mLErlenmeyer flask, which was closed off by means of a septum coated withaluminum foil, so that a gas phase formed above the liquid. A glass rodwhich was encased by 50 mg of polydimethylsiloxane (PDMS) as pushed-onhose 110, was held in the gas phase as inert support 109; the hose 110surrounded the inert support 109 over a length of 5 cm and with a wallthickness of 0.2 mm. The enclosed glass rod was introduced into athermal desorption apparatus by means of a small thermal desorption tubeand was subjected to thermal desorption, during which the thermaldesorption tube, which had been heated to 40° C., was heated insplitless mode to 250° C. with a heating rate of 60° C./min, and thedesorbed substances to be investigated were then transferred to a coldcharging system by means of carrier gas and via a transfer line. In thecold charging system, in this case a Tenax-filled quartz liner, thesubstances to be investigated were received at −150° C. and thenreleased again using a heating rate of 600° C./min up to a finaltemperature of 250° C. With a split ratio of 1:50, in order to obtainsignal peaks which are suitable for the quantity of sorbed substances.The Tenax-filled quartz liner in the cold charging system prevents thehighly volatile components from breaking through. The substances to beinvestigated were then transferred to a GC column by means of a carriergas; in this example, a GC column which was 30 m long, had an internaldiameter of 250 μm and a film thickness of 1 μm was used. The GC columnwas heated by a furnace which surrounds it and initially held thetemperature at 30° C. for 1 min and then raised it to 300° C. with aheating rate of 10° C./min. The substances to be investigated wereanalyzed using an MSD 5973 (mass-selective detector) in scan mode in therange from 20-400 amu (atomic mass units).

In the comparative example using the known SPME method, the specimen wasprepared in a similar manner and a fiber coated with PDMS in a layerthickness of 100 μm was held in the gas phase in the closed Erlenmeyerflask. This layer thickness approximately corresponds to a coatingquantity of 0.5 mg of PDMS. The thermal desorption and cold charginglikewise took place in a similar manner to the HSSE method, but thephase of heating up the cold charge and the subsequent transfer to theGC column was only able to take place in splitless mode, on account ofthe small quantities of substance present, while the subsequent analysiswas once again carried out in a similar manner to the HSSE method.

FIG. 12 shows a comparative diagram illustrating the yield (plotted onthe ordinate) for the hose using the HSSE method of the example and forthe coated fiber of the SPME method of the comparative example; theretention time of the substances investigated is plotted on theabscissa. If the retention time is approximately 10 min, it can be seenfrom the diagram shown in FIG. 12 that, in this case, the coated fiberof the SPME method gives a yield of virtually 0%, while the HSSE methodgives a yield of approximately 6%. With retention times of less than 10min, the coated fiber of the SPME method generally does not allow areliable measurement to be carried out (cf. also FIG. 14), while thehose generally allows reliable measurements (cf. also FIG. 13). Overall,the comparative diagram of FIG. 12 demonstrates that, with the hose ofthe HSSE method, the yield is improved considerably, i.e. by at leastone power of 10, and the measurement range is widened considerably,since the analysis sensitivity is improved by approximately a factor of50.

FIG. 13 shows a GC spectrum using the HSSE method, in which theintensities are plotted against the retention times; this spectrum isobtained by means of the example described above and constitutes thebasis for the comparative diagram shown in FIG. 12.

FIG. 14 shows a GC spectrum using the SPME method, in which theintensities are plotted against the retention times; this spectrum isobtained by means of the comparative example described above andconstitutes the basis for the comparative diagram shown in FIG. 12.

A comparison of FIG. 13 and FIG. 14 shows that overall the HSSE methodreveals a significantly improved sensitivity and lower retention timeswith, at the same time, a considerably higher intensity, which in thisexample, moreover, emerges particularly clearly for substances with lowmolecular masses as well.

Although the foregoing has been a description of preferred embodimentsof the invention, it will be apparent to those skilled in the art thatnumerous variations and modifications may be made in the inventionwithout departing from the scope as described herein.

1-51. (canceled)
 52. A collector for the solid-phase microextraction andanalysis of substances to be examined, comprising a rod-like elementsheathed in a flexible tube of material selected from the groupconsisting of a sorbent, an adsorbent and a combined sorbent andadsorbent, and capable of being picked up by a discharging deviceselected from the group consisting of a gripping device, suction device,and a magnet, and removed.
 53. The collector of claim 52, adapted to beinsertable in a thermal desorption apparatus of a gas chromatograph. 54.The collector of claim 52 wherein said material is a member selectedfrom the group consisting of polyethylene glycol, silicone, polyimide,octadecyltrichlorosilane, polymethylvinylchlorosilane, liquid-crystalpolyacrylates, grafted self-organized monomolecular layers and inorganiccoating materials.
 55. The collector of claim 52 wherein said rod-likeelement is a section of wire.